1. Field of the Invention
The present invention generally relates to a method for manufacturing a wiring pattern board. More specifically, the present invention is directed to a method for manufacturing a wiring pattern board, by which solder can be supplied onto a mounting pad arranged in a narrow pitch without a formation of a bridge, and also with less thickness fluctuations.
2. Description of the Related Art
Very recently, there is a trend to manufacture printed wiring pattern boards in high density and to make very narrow pitches of LSI mounting pads, while electronic appliances are made more compact and in higher performance. In general, as methods for manufacturing printed boards, either the subtractive process or the additive process is applied, as disclosed in Japanese Laid-open Patent Application No. 5-335734.
In the subtractive process, a desirable wiring pattern is formed on a stacked layer covered with copper by way of an etching process, and then a wiring pattern portion from which a mounting pad has been removed is protected by forming solder resist on this wiring pattern portion.
On the other hand, in the additive process, permanent resist is formed on a base member with catalyst in order to obtain a desirable wiring pattern, and then non-electrolytic copper plating is performed on the base member so that the desirable wiring pattern is directly formed. Thereafter, solder resist is formed on a wiring pattern portion from which a mounting pad has been removed in a similar manner to the subtractive process.
Then, as a method for supplying solder used to mount an LSI on the conventional printed wiring pattern board obtained by way of any one of the above-described subtractive and additive processes, for example, there is a solder paste printing process with employment of a metal mask. This printing process includes a limitation in supplying the solder to the mounting pad with the pad pitch of 250 to 300 .mu.m, because there are printing shifts and limitations in forming of the metal mask opening.
Accordingly, the improved processes such as the SUPER SOLDER process and the SUPER JUFFIT process have been conventionally proposed which may overcome problems associated with the solder paste printing process. The SUPER SOLDER process is featured by using such solder paste which may react only the copper pad during the heating reaction. In this conventional process, such a metal mask may be applied that the openings in every pad are not formed, but one edge of the pad is completely printed out.
Then, the SUPER JUFFIT process is featured by using flux for fixing solder powder only on the copper pad. Similar to the SUPER SOLDER process, such metal masks may be applied that the openings every pad are not formed but one edge of the pad is completely printed out.
The previous two solder supplying processes include a feature that the solder can be supplied onto the mounting pads arranged with relatively narrow pitches without producing bridges. However, the amount of solder supplied onto each of the pads is not constant. As a result, there is a problem that the amount of solder, namely the height of solder, fluctuates between pads. Also, in these processes, the applicable amount of the solder is determined based on the surface tension defined by the pad shape. As a consequence, if the pitches are made further narrow as in the bare chip mounting pad and thus the pad width becomes narrow, then the amount of the supplied solder is accordingly lowered. Then, there is another problem that the amount of solder required for mounting purposes cannot be secured.
To solve these problems, another conventional method has been proposed in which a preselected amount of solder piece is positioned every pad, and thereafter this solder piece is melted by the reflow to supply the solder.
In the MICROPRESS process disclosed in Japanese Laidopen Patent Application No. 4-120735, the solder is punched out by the punch 7 with respect to each of the mounting pads, the punched solder is connected under pressure, and then the connected solder is melted to thereby supply the melted solder.
In the MICROBALL process disclosed in Japanese Patent Publication No. 7-19799, a very small solder ball is transferred to the mounting pad and then is melted to thereby supply the melted solder.
According to these two process, after a preselected amount of solder has been fixed to the pads arranged in the narrow pitch, this fixed solder is melted by the reflow.
There is another problem in the conventional printed wiring board manufacturing methods that the supplied solder is dropped out from the pad before being melted.
That is, the pad shape of the conventionally used printed wiring board becomes convex in the subtractive process, or flat in the additive process, whereas these shapes are made in such a way that the solder piece and the solder ball can hardly be fixed together. Therefore, there is a problem that the solder piece or the solder ball positioned on the pad cannot be sufficiently fixed on this pad, and therefore is dropped from this pad when vibrated.
Moreover, there is another problem, in the conventional printed wiring board manufacturing methods, such that when a large amount of solder is supplied to each of these plural pads arranged in the narrow pitches (for instance, not more than 120 .mu.m pitch), the bridges are produced. In other words, when a large amount of solder is supplied to a plurality of pads arranged in the narrow pitches, the surface tension can no longer be maintained during the reflow melting operation, and thus the solder will flow along the side surface direction of the pad. As a consequence, there is a further problem that the bridges are produced.